(1) Field of the Invention
This invention relates to an enhanced oil recovery process for subterranean reservoirs, especially those which have heterogeneous permeability. More particularly, the invention relates to such a process wherein displacement of oil and/or gas through the reservoir is accomplished in a more uniform manner.
(2) Description of the Prior Art
Most subterranean oil-bearing reservoirs have a heterogeneous permeability at least in some degree. Thus, fluids passing through such reservoirs tend to pass through different portions of the reservoir at different rates. Generally, the higher the permeability of a given portion or zone of a reservoir the higher the flow rate through that portion. If this difference in permeability is sufficiently high, substantially all of the fluid passes through only the more permeable portions of the reservoir. Thus, little fluid moves through the less permeable portions which may constitute the bulk of the reservoir.
In enhanced oil recovery processes, a displacing fluid is injected into a reservoir via one or more injection wells to displace oil through the reservoir to one or more spaced production wells through which oil is produced. One of the principal objectives of such processes has always been to achieve the maximum displacement of reservoir oil by the displacing fluid. In order to accomplish this, it is necessary to establish a uniform flow of fluids through the reservoir.
In conjunction with enhanced oil recovery processes carried out in heterogeneous oil-producing reservoirs, it has been the practice to treat the reservoir at some time during the enhanced oil recovery process by injecting therein via a single injection well or a single production well a solution that enters the more permeable zones of the reservoir surrounding the well and sets after the passage of time to form a plug. U.S. Pat. No. 3,581,824 to Hurd describes an oil recovery process wherein there is injected through an injection system first an aqueous solution containing divalent cations and next an aqueous solution of an ionic polysaccharide thickening agent which sets up upon contacting the divalent cations to plug the more permeable zones of the formation. The polysaccharide solution is followed by a driving fluid. Oil is recovered via a production system. The injection and production systems may each comprise one or more wells and such wells may be located and spaced from one another in any desired pattern. While for simplicity in describing the invention, only one injection well and one production well are shown in the drawing, in practical application of the invention, a plurality of wells may be, and in most cases will be, utilized.
Thus, in the Hurd process it has been the practice to form only a single plug between injection and production wells. If channeling of fluids through the reservoir occurs at some time following placement of such a plug, it is known to inject additional plug-forming solutions in a manner similar to the way in which the first plug was formed. However, due to the wide variations in the character of heterogeneous formations, success of the treatments described above have often been limited. The most permeable channel may be plugged but the fluids are merely diverted into the next most permeable channel or the plug in the most permeable channel is bypassed and uniform flow of fluids through the reservoir is still not achieved.
U.S. Pat. No. 3,386,509 to Froning describes a method of plugging all or substantially all of a high permeability zone between two wells. A settable silicate solution is injected via a first well for part of or the entire distance between the two wells, e.g., to fill from about 50 percent to about 90 percent of the permeable zone. Next, a plug is formed in the zone near the second well by injecting therein either a solution of an activator or a settable mixture. While treatment of high permeability zones in the above-described manner has met with some success, there has been an indication that, in some instances, it is desirable to form plugs of a more limited extent in the high permeability zones between wells. Not only does this result in a savings in the quantity of plug-forming composition required, but also enables placement of a satisfactory plug without undue damage to the production and/or injection potential of a well. Preferably, precautions should be taken to confine the extent of the plugs to the area in the immediate vicinity of each well. Surprisingly, the small plugs formed in the vicinity of each well enables an enhanced oil recovery fluid to displace hydrocarbons from the reservoir in a more uniform manner than can be achieved if the plugs are made larger, i.e., occupy a greater fraction of the high permeability zones.
Accordingly, a principal object of this invention is to provide a process for maximizing the recovery of oil from a heterogeneous reservoir during an enhanced oil recovery method.
A further object of this invention is to provide such a process wherein fluids flow through all zones of the reservoir at a more uniform rate.
A still further object of this invention is to provide such a process wherein channeling of fluids through the reservoir is decreased.
Another object of this invention is to provide such a process wherein the more permeable zones of the reservoir are more effectively plugged.
Still another object of this invention is to form a plug at each end of a channel communicating with a well.
Still another object of this invention is to provide such a process wherein the position of each plug formed is controlled.
Still another object of this invention is to provide such a process wherein the extent of each plug formed is limited and controlled.
Other objects, advantages and features of the invention will become apparent from the following detailed description and drawing.